Genetics: Mutations

Genetic mutation
Genetic mutation

A mutation is a heritable, sudden change in the structure of a gene, which has no relation to the individual’s ancestry. The change can occur spontaneously or can be the result of exposure to ultraviolet radiation or chemicals.

In 1901 Hugo de Vries coined the term mutation to describe changes in the hereditary material of evening primrose (Oenothera). “Mutation” is a derivative of the Latin verb mutare, meaning “to move or change.” The word was first used to describe spontaneous, heritable changes in the phenotype of an organism.

In the modern era of genomics, mutations can be defined as changes in DNA (deoxyribonucleic acid) sequences, that is, changes in the structure of a gene. The changes can occur spontaneously or can be induced via ionizing radiation (ultraviolet radiation) or chemicals, such as aflatoxin B1 and ethylmethane sulfonate.

A common cause of spontaneous mutations is deamination, in which the amino group on the number 2 carbon of cytosine (C) is removed, converting C to uracil (U) in DNA. Another cause is copying errors during DNA replication: slippage or shifting of the translational reading frame.

Spontaneous mutations also may be caused by depurination, in which the bond between deoxyribose sugar and a purine base, adenine (A) or guanine (G), is hydrolyzed, or by depyrimidination, the hydrolization of the bond between deoxyribose sugar and a pyrimidine base, either Cor thymine (T).

Depyrimidination is less common than depurination. The sites where a base is missing are called apurinic sites (when a purine base is missing) or apyrimidinic sites (when a pyrimidine base is missing) or simply AP sites.

An individual with a mutation is called a mutant. When a mutation occurs in the reproductive tissue of an individual plant, it can be transmitted to the next generation. When a mutation occurs in the somatic tissue, it will be limited only to that generation and affects only the cells in which it occurs.

Heredity vs. Genetic Combinations

Mutant strawberry
Mutant strawberry

Mutation is the only process that creates new genetic variation that results from a change or changes in the structure of a gene or genes. Genetic variability also can arise from new genetic combinations produced through the processes of crossing over, gene segregation, and chromosome assortment.

In the case of recombination, all the genes are already present in an individual, and new variation simply results from the shuffling of those genes during gamete formation; there are no structural changes in genes.

Sites and Types of Mutation

Gene mutation involves a change in a single base pair or a deletion of a few base pairs. It usually affects the function of a single gene. The substitution of one base (or nucleotide) for another base (or nucleotide) is called a point mutation or a substitution mutation.

The replacement of a pyrimidine (cytosine and thymine) with another pyrimidine or the replacement of a purine (adenine and guanine) with another purine is termed transition. The replacement of a pyrimidine with a purine or the replacement of a purine with a pyrimidine is termed transversion.

Base pair or nucleotide changes can produce one of the following types of mutation:
  • Missense mutation, which results in a protein in which one amino acid is substituted for another amino acid.
  • Nonsense mutation, in which a stop codon is substituted for an amino acid codon, which results in premature termination of a protein.
  • Frameshift mutation, which causes a change in the reading frame. These mutations can introduce a different amino acid into the protein and have a much larger effect on protein structure. Small deletions also have effects similar to those of frameshift mutations. A nonfunctional protein may be produced, unless the frameshift is near the terminal end of a gene.
  • Chromosomal mutations or abnormalities, which involve deletions or insertions of several contiguous genes, inversion of genes on a chromosome, or the exchange of large segments of DNA between non homologous chromosomes.

Effects of Mutations

Gene flow is the exchange of genes between different populations of the same species, produced by migrants and commonly resulting in changes in gene frequencies at many loci (locations) in the recipient gene pool. The effect of a mutation carried into a new population will depend upon the number of migrants that were mutants and the size of the recipient population.

Genetic drift is the random fluctuations of gene frequencies as a result of sampling errors. Drift occurs in all populations, but its effects are most striking in small populations. Due to periodic reductions in population size, genetic drift can affect gene frequencies.

A large population may contract and expand again with an altered gene pool (called the bottleneck effect). A consequence of genetic drift is reduced variability. A mutation has a better chance of spreading faster in a smaller population than in a larger population.

The “founder effect” is a term first coined by Ernest Mayr in 1942, who referred to small groups of migrants that succeed in establishing populations in a new place as “founders.” Two founders could carry only four alleles at each gene locus. If a rare (mutant) allele were included among them, its frequency would be considerably higher (0.25) than it was in the parental population.

Nonrandom mating, also called assortative mating, occurs when male and female plants are not crossed at random. If the two parents of each pair tend to be more (or less) alike than is to be expected by chance, then positive (or negative) assortative mating occurs.

Positive assortative mating promotes homozygosity, whereas negative assortative mating tends to promote heterozygosity. Mating of similar homozygotes would increase their frequency at the expense of heterozygotes. A mutation should spread more quickly under assortative mating than under nonassortative mating.

Inbreeding is defined as the coming together, at fertilization, of two alleles that are identical by descent. This is the result of mating between closely related plants.

A mutation has a better chance of establishing under mating systems in which close relatives are involved than under those where in breeding is prevented. Assortative mating represents the mating of individuals with similar phenotypes, whereas inbreeding represents the mating of individuals of similar genotypes.

Autogamy, or self-fertilization, is the strictest formof inbreeding. A mutation would spread more quickly under self-pollination than under cross-pollination.

If a mutation occurs at a homozygous locus (aa × Aa, or AA × Aa), the result would be greater diversity. If a mutation occurs at a heterozygous locus (Aa × AA or aa), it would result in more uniformity. Depending on the size of the population, gene frequency will change.